1. Field of the Invention
This invention relates to a directional coupler type optical functional device formed of semiconductor material and a method of driving the same, and more particularly to an optical functional device useful as an optical switch and an optical mode splitter and a method of driving the same.
2. Prior Art
Recently, various waveguide types of optical functional devices are proposed. However, most of the devices can be operated only for light which is polarized in a specified direction. For this reason, in the optical fiber communication which is now put into a practical stage, the above devices cannot be practically used unless light is previously subjected to the polarization control.
Under this circumstance, various studies on a polarization separator and a polarization-independent optical switch are actively made. Examples of the studies are explained with reference to the drawings below.
First, FIG. 1 is a schematic perspective view of a device disclosed in the article by Tadasu Sunada et al. in "ELECTRON INFORMATION COMMUNICATION INSTITUTE PAPER (DENSHI JOHO TSUSHIN GAKKAISHI)", C-I (vol J73-C-I. No. 9. pp559 to 566, September, 1990).
The device is constructed by forming a nonsymmetrical X-branch optical waveguide 2 by use of Ti-doped SiO.sub.2 on an LiNbO.sub.3 substrate 1, disposing three electrodes 3 in positions shown in the drawing and applying voltages between the respective electrodes as shown in the drawing, and the device may function as a polarization-independent optical switch and a TE mode/TM mode splitter.
FIG. 2 is a schematic plan view of a device proposed by J. Saulnier et al. in ECOC90-229 (1990) and the device is constructed by forming an optical waveguide by use of Ti-doped SiO.sub.2 on an LiNbO.sub.3 substrate, disposing electrodes (hatched portions) as shown in the drawing and applying voltages to the respective electrodes as shown in the drawing.
A device shown in FIG. 3 and FIG. 4 which is a cross sectional view taken along the line IV--IV of FIG. 3 and proposed by M. Okuno et al. in "Photonic Switching" pp38 to 40, 1990 is constructed by embedding cores 5 formed of SiO.sub.2 into a clad 6 on an Si substrate 4 to form a directional coupler type optical waveguide 7, depositing a thin Cr film to form a current injection electrode 8 in position of one of the waveguides shown in the drawing, and forming an a-Si thin film 9 in position of the other waveguide shown in the drawing.
As shown in FIG. 5, an optical mode splitter proposed by M. Kobayashi et al. in Appl. Phys. Lett., vol 32, pp300 to 302 (1978) is known.
The above optical mode splitter has an optical waveguide which is formed of glass-series material of SiO.sub.2 -Ta.sub.2 O.sub.5, an waveguide layer 11 and an interlaid layer 12 which are sequentially disposed as a 2-dimensional waveguide on a substrate 10, and a thin waveguide layer 13 which is formed with a preset inclination angle on the interlaid layer 12, and functions to receive incident light in a Z direction in the drawing and separates the same into the TE mode and TM mode. That is, when light is made incident on the waveguide layer 11 in the Z direction, the mode selection condition established by the inclination of the thin-film waveguide layer 13 is satisfied, a TE mode component 14a of the incident light is coupled with the thin-film waveguide layer 13, and a TM mode component 14b thereof travels straight without being coupled with the thin-film waveguide layer 13. As a result, the TE mode 14a and the TM mode 14b can be separated.
Since all of the portions of each of the above-described four types of devices are not formed of semiconductor material, it is impossible to apply the above devices to an optical integrated circuit device of polarized diversity light reception system manufactured by integrating an active element such as an LD, LED or PD having most portions formed of semiconductor material in a monolithic form. Further, the optical mode splitter shown in FIG. 5 is not suitable for integration and it is difficult to deposit the waveguide layer, interlaid layer and thin-film waveguide layer at a high precision, and as a result, it becomes difficult to properly attain the mode coupling condition in the vertical direction (thickness direction) of the device.
Furthermore, the above devices are large in size, significantly low in the characteristics thereof in response to temperature changes, and are slow in the response speed, and because of high power consumption, they are liable to suffer optical damage and DC drift. In order to solve the above problems and make it possible to integrate the active elements in a monolithic form, all the portion of the device may be formed of semiconductor material.
Before introducing optical functional devices having entire portions formed by semiconductor material, conventional optical splitter devices will be explained firstly.
FIG. 6 and FIG. 7 which is a cross sectional view taken along the line VII--VII of FIG. 6 show an optical mode splitter proposed by M. Masuda and G. L. Yip in Appl. Phys. Lett., vol 37, pp 20 to 22 (1980).
In the optical mode splitter, a Y-branch multiple mode waveguide is first formed by use of LiNbO.sub.3 -series material (FIG. 6). Then, as shown in FIG. 7, a buffer layer 16 such as an Al.sub.2 O.sub.3 layer is laid in part of a main path 15 and one of the waveguides (15b in the drawing) so that the equivalent refractive indices thereof can be made different from each other and one set of electrodes 17a and 17b are disposed so that a voltage can be applied between the electrodes as shown in FIG. 7.
Assume now that the equivalent refractive index of the branch waveguide 15a is n.sub.1, the equivalent refractive index of the branch waveguide 15b is n.sub.2, and n.sub.1 &lt;n.sub.2.
In this state, light in which both of the TE mode and TM mode are present is made incident on the main path 15. Since the equivalent refractive index of the branch waveguide 15b is larger than that of the branch waveguide 15a, the incident light is confined in the branch waveguide 15b and emitted therefrom.
However, if a voltage is applied between the electrodes 17a and 17b, the equivalent refractive index of the branch waveguide 15b is lowered only for the TE mode by the electro-optical effect. Therefore, if application of the voltage causes the amount of reduction in the refractive index to become larger than .vertline.n.sub.1 -n.sub.2 .vertline., and when a higher voltage is applied, the TE mode component of the light incident on the main path 15 is confined in the branch waveguide 15a whose equivalent refractive index is made larger than that of the branch waveguide 15b and emitted therefrom. Since the equivalent refractive indices of the branch waveguides 15a and 15b are kept unchanged for the TM mode component of the incident light, the TM mode light travels in the branch waveguide 15b and is emitted therefrom. Thus, the incident light can be split into the TE mode and TM mode by application of the voltage between the electrodes 17a and 17b.
However, in the case of the above optical mode splitter, the integration is difficult, the structure thereof is improper for mode communication, the mode splitting efficiency is low, and a high extinction ratio cannot be attained.
FIG. 8 is a schematic perspective view of an optical mode splitter proposed by M. Erman et al. in 15th ECOC.ThB201 (1989).
In the above optical mode splitter, a directional coupler type optical waveguide is constituted by two waveguides 18a and 18b arranged in parallel using semiconductor material and the upper surface of one of the waveguides (18b in the drawing) is covered with a metal layer 19.
With the above construction, the equivalent refractive indices for the TE mode and TM mode are made different between the waveguides 18a and 18b.
Therefore, when light in which both of the TE mode and TM mode are present is made incident on the waveguide 18a, the TE mode light is coupled with the waveguide 18b which is covered with the metal layer 19 and the TE mode light is guided in the waveguide 18b and emitted therefrom. However, since the TM mode light is not coupled with the waveguide 18b, it is emitted from the waveguide 18a as it is. That is, the TE mode and TM mode are separated from each other.
However, since the above optical mode splitter does not function unless the length of the coupling portion is equal to the complete coupling length for the TE mode, it becomes necessary to form the coupling portion at an extremely high precision in order to attain the function. However, it is difficult to form the coupling portion with a required high precision by use of the present photolithographic technology and etching technology and actually manufactured devices cannot meet the requirement of high dimensional precision so that the mode splitting efficiency will become low and a high extinction ratio cannot be attained.
Further, a device with the construction shown in FIG. 9 and FIG. 10 which is a cross sectional view taken along the line X--X of FIG. 9 is disclosed in Published Unexamined Japanese Patent Application No. 2-170103.
The device includes a diffraction grating 22 disposed in an orthogonal section 21 of emission side optical waveguides 20a and 20b which cross at right angles and part of the upper surface of the emission waveguide 20a which is one of the emission side optical waveguides is covered with a metal layer 23, and it functions as an optical branching filter for separating the TE mode and TM mode from each other.
In the case of the above device, the extinction ratio thereof may be approximately several tens dB although not clearly determined. Further, the mode separation depends on the uniformity of the depth of a groove of the diffraction grating 22 introduced into the orthogonal section 21. In view of the manufacturing process, since it is extremely difficult to control realization of the uniformity of the depth and the emission side optical waveguides 20a and 20b of both modes are crossed at right angles, inconvenience may occur when it is integrated together with another element and connected to the same.
Next, a conventional optical switch which is of directional coupler type and is formed of semiconductor material is explained.
FIG. 11 is a schematic plan view of an optical switch of uniform .DELTA..beta. structure. In the case of the above optical switch, two optical waveguides 24 and 25 formed of semiconductor material are arranged on a semiconductor substrate (not shown) and an optical waveguide section 24a and an optical waveguide section 24b are arranged closely to and in parallel with each other so as to be evanescent-coupled so that a coupling portion can be formed in an area A surrounded by broken lines in the drawing.
An electrode 26 is formed on one of the optical waveguide sections (24a in the drawing) and voltage application and current injection with respect to the optical waveguide section 24a can be effected by use of the electrode 26.
In the case of the optical switch, for example, if light is made incident on an upstream side end portion 25b of the optical waveguide 25 and the electrode 26 is set in the non-driven state, the light is coupled with the optical waveguide section 24a in the coupling portion A and emitted from a downstream side end portion 24c of the optical waveguide 24. That is, the light incident on the upstream side end portion 25b is emitted from the downstream side end portion 24c of the optical waveguide 24 and is not emitted from the downstream side end portion 25c of the optical waveguide 25.
However, for example, when current is injected via the electrode 26 to lower the equivalent refractive index of the optical waveguide section 24a, light incident on the upstream side end portion 25b of the optical waveguide 25 passes in the optical waveguide section 25a and is emitted only from the optical waveguide section 25a without being coupled with the optical waveguide section 24a. That is, by current injection via the electrode 26, the light emission side is changed from the downstream side end portion 24c to the downstream side end portion 25c, thus attaining a switching function.
However, in order to operate the uniform .DELTA..beta. structure type optical switch, it is necessary to previously adjust the emission ratio of the downstream side end portions 24c and 25c of the two optical waveguides 24 and 25 to 1:0 (or 0:1). This can be attained by precisely controlling the lengths of the optical waveguide sections 24a and 25a and a distance between the optical waveguide sections in the coupling portion A. However, it is extremely difficult to set the length of the coupling portion A at a high precision with the present level of photolithographic technology. For this reason, in the case of the above uniform .DELTA..beta. structure type optical switch, crosstalk will inevitably occur in the process of optical coupling.
An reversal .DELTA..beta. structure type optical switch shown by a schematic plan view of FIG. 12 is proposed to solve the above-described problem of the uniform .DELTA..beta. structure type optical switch.
In the coupling portion A of the optical switch, electrodes 26a and 26b are disposed to be symmetrical with respect to a point on a downstream side portion 24d of an optical waveguide section 24a and the upstream side portion 25d of an optical waveguide section 25a and the electrodes 26a and 26b are connected to each other via a connecting portion 26c. With this construction, the electrode 26a may be disposed on an upstream side portion 24e of the optical waveguide section 24a and the electrode 26b may be disposed on a downstream side 25e of the optical waveguide section 25a.
Unlike the uniform .DELTA..beta. structure type optical switch, in the optical switch of the above construction, the coupling state will not be restricted by the initial condition of the coupling portion A.
First, when the electrodes 26a and 26b are set into the non-driven state and light is made incident on the upstream side end portion 25b of the optical waveguide 25, for example, then the light is coupled with the optical waveguide section 24a and confined therein in the coupling portion A and is emitted from the downstream side end portion 24c.
Next, when voltage application or current injection is effected with respect to the electrode 26a (26b), a cross state is established between the optical waveguide sections 24a and 25a at a certain voltage or current, and if the voltage or current is further increased, a switching state appears after a thorough state has appeared.
That is, .DELTA..beta. in the entire portion of the coupling portion A is reversed by voltage application or current injection by means of the electrode 26a (26b) and the emission end for light incident on the upstream side end portion 25b is changed from the downstream side end portion 24c to the downstream side end portion 25c.
In the case of the reversal .DELTA..beta. structure type optical switch, the coupling state can be reliably controlled irrespective of the initial condition of the coupling portion A. However, in general, the value of the voltage or current required for establishing the through state becomes large although the value of the voltage or current required for establishing the cross state is not so large.
Therefore, the pn junction formed in the optical waveguide may be damaged by heat generation caused by large power consumption in the semiconductor material constituting the optical waveguide, thereby reducing the service life of the element.